feat: add browser RPN calculator engine and demo

.continue/rules/project.md

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+# Project rules — RPN Virtual Calculator
+
+- Build a browser-friendly RPN calculator as a JavaScript class, preferably in a single file.
+- Constructor options: `maxSize` (default 2048), `base` (default 10), `angleMode` (`deg` default; also `rad` and `grad`), `enabledCommands`.
+- Available constants: `pi`, `e`.
+- Public API is generic only: `push`, `pop`, `clear`, `swap(index1, index2)`, `remove(index)`, `edit(index)`, `isValidIndex(index)`, `input(command)`, and `command(name, ...args)`.
+- Expose `inputValue` as a string and `isEditing` as a boolean.
+- Supported operations are centralized in one dictionary with `argCount`, category, and aliases.
+- Categories are limited to `Stack`, `Arithmetic`, and `Trigonometry`.
+- Current commands: `add`, `sub`, `mul`, `div`, `mod`, `pow`, `sqr`, `neg`, `sqrt`, `recip`, `sin`, `cos`, `tan`, `asin`, `acos`, `atan`, `log`, `ln`, `dup`, `drop`, `swap`, `clear`, `enter`.
+- Aliases: `+`, `-`, `*`, `/`, `%`, `^`, `y^x`, `1/x`.
+- `mod` is the percentage operator: `a b % => (a * b) / 100`.
+- `sqrt`, `asin`, `acos`, `log`, and `ln` must throw clear, explicit domain errors.
+- `log` uses `Math.log10`; `ln` uses `Math.log`.
+- Trig functions use degrees in the demo; `sin`, `cos`, `tan` convert to radians, inverse trig returns degrees.
+- `inputValue` stays a string to keep hexadecimal input possible later.
+- Example HTML must group buttons by `Stack`, `Arithmetic`, and `Trigonometry`, and call `command(...)`.
+- Keep code names, categories, and API identifiers in English.
+- Keep this file updated after each project change using the provided editing tools.
+